Fischer: Know your valve’s limitations 

Robert L. Fischer, P.E., is a physicist and electrical engineer who spent 25 years in chemical plants and refineries. Fischer can additionally be a part-time school professor. He is the principal reliability marketing consultant for Fischer Technical Services. He may be reached at
One of Dirty Harry’s famous quotes was: “A man’s got to know his limitations.” This story illustrates why you should know your management valve’s limitations.
A client just lately known as for help downsizing burners on a thermal oxidizer. Changes within the manufacturing course of had resulted in an excessive quantity of warmth from the present burners. All makes an attempt to decrease temperatures had ended in unstable flames, flameouts and shutdowns. The larger temperatures didn’t hurt the product however the burners have been guzzling 110 gallons of propane each hour. Given the high cost of propane at that plant, there have been, actually, tens of millions of incentives to conserve power and reduce prices.
Figure 1. Operation of a cross connected air/gas ratio regulator supplying a nozzle combine burner system. The North American Combustion Practical Pointers guide could be discovered online at pressure gauge 10 bar , Inc. 4455 East 71st Street, Cleveland, OH 44015. Image courtesy of Fives North American Combustion, Inc.
digital pressure gauge to retrofit smaller burners was being written. One of the plant’s engineers called for a worth estimate to change burner controls. As we mentioned their efforts to scale back gasoline usage, we realized smaller burners may not be required to unravel the problem.
Oxidizer temperature is basically determined by the place of a “combustion air” management valve. Figure 1 shows how opening that valve increases strain in the combustion air piping. Higher stress forces extra air by way of the burners. An “impulse line” transmits the air strain to one facet of a diaphragm in the “gas control valve” actuator. As air strain on the diaphragm increases, the diaphragm moves to open the valve.
The gasoline valve is routinely “slaved” to the combustion air being provided to the burner. Diaphragm spring tension is adjusted to ship the 10-to-1 air-to-gas ratio required for stable flame.
The plant was unable to take care of flame stability at significantly lower fuel flows as a result of there’s a limited range over which any given diaphragm spring actuator can present correct management of valve position. This usable management range is identified as the “turndown ratio” of the valve.
In this case, the plant operators no longer needed to totally open the gas valve. They needed finer resolution of valve place with a lot lower combustion air flows. The diaphragm actuator needed to be able to crack open after which control the valve utilizing considerably lower pressures being delivered by the impulse line. Fortunately, changing the spring was all that was required to allow recalibration of the fuel valve actuator — using the prevailing burners.
Dirty Harry would positively approve of this cost-effective change to the valve’s low-flow “limitations.” No capital challenge. No burner replacements. No significant downtime. Only a quantity of inexpensive elements and minor rewiring were required to keep away from wasting “a fistful of dollars.”


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